(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming gate electrodes and contact metallization to self-aligned-gate field effect transistors.
(2) Background of the Invention and Description of Prior Art
Refractory metals and their suicides have found widespread use in the manufacture of very large scale integrated circuits. Tungsten(W), in particular, because of its chemical stability and its ease of film formation by chemical vapor deposition(CVD), is used extensively for the formation of electrical contacts to silicon device elements. Tungsten silicide(WSi.sub.2) is frequently selectively formed over exposed silicon surfaces by depositing a tungsten layer, annealing to react the layer with the silicon areas and etching away the unreacted tungsten, leaving tungsten silicide only on the silicon.
As a conductive material tungsten does not rank as high as aluminum, which has been the primary conductor used in micro-circuit chip technology for nearly forty years. On the other hand, tungsten and some of its compounds offer many features which make them important materials for fabricating metal-to-silicon contacts, via plugs, polycide gate electrodes, and intra/inter-level interconnects. CVD tungsten has low stress (less than 5.times.10.sup.9 dynes/cm.sup.2), a coefficient of thermal expansion which closely matches that of silicon, and a high resistance to electromigration which is a common problem with aluminum its alloys. CVD tungsten can be deposited at temperatures around 400.degree. C. using the hydrogen or silane reduction of tungsten hexafluoride(WF.sub.6) with good conformity and step coverage.
Over time the art of tungsten deposition has been developed to provide optimal combinations of properties for specific applications. In many instances this has resulted in making combined use of both the hydrogen and silane reduction process as well as performing the deposition in stages whereby such factors as adhesion, grain structure, edge coverage, sheet resistance and stress are balanced to provide high quality tungsten contact plugs and interconnects. Emesh, U.S. Pat. No. 5,407,698 and Chang, et.al. U.S. Pat. No. 5,028,565 cite examples of such optimizations, wherein the gas flows introduced into the reactor contain both hydrogen and silane. The fluorine containing species produced by the reduction of WF.sub.6 to form tungsten conductive films have heretofore made their application directly onto gate oxides impractical. Gate oxides exposed to these species are seriously degraded not only by fluoride incorporation into the oxide but also by fluorine induced oxide deterioration.
When W is deposited into contact openings adjacent to silicon oxide gate layers, the HF by-product of the hydrogen reduction process is believed to be responsible for problems of encroachment under the oxide as well as for wormholes which lead to junction leakage. This is discussed by Wolf, S., "Silicon Processing for the VLSI Era", Vol.2, Lattice Press, Sunset Beach, Calif., (1990), p.247. In addition residual fluorides left by the W deposition can become corrosive to nearby metal films such as aluminum. Although HF is not directly formed by the silane reduction reaction, the hydrogen by-product can subsequently react with WF.sub.6 to form it.
Generally, metal layers are deposited either by physical vapor deposition(PVD) method such as sputtering or vacuum evaporation or by low pressure CVD (LPCVD) or Atmospheric pressure CVD (APCVD). Recent efforts have been successful in depositing metals such as aluminum and copper from metal-organic precursors using metal-organic CVD (MOCVD). Tungsten and molybdenum gate electrodes have been formed by magnetron sputtering as reported by Wolf, S., "Silicon Processing for the VLSI Era", Vol.2, Lattice Press, Sunset Beach, Calif., (1990), p398. These materials have significantly lower conductivities than polysilicon or polycide gates.
Price, U.S. Pat. No. 4,692,343 cites the formation of tungsten silicide by a plasma enhanced CVD (PECVD) process using tungsten hexafluoride (WF.sub.6) and dichlorosilane (SiH.sub.2 Cl.sub.2). The apparatus consists of an ante-chamber in which a plasma is ignited and a chamber in which deposition takes place. The plasma discharge is applied only to initiate deposition of tungsten silicide. Sustaining the plasma past the initial period was found to be detrimental to the deposition. The problem with this reactor configuration is that sustaining a plasma would cause film deposition over the walls of the upper chamber thereby depleting the reactant gas as well as contaminating the chamber. Thus plasma activated species would not be available during the film deposition period.
Jucha, in U.S. Pat. No. 4,838,990 shows a method for plasma etching tungsten and in U.S. Pat. No. 4,874,723 shows a process of etching tungsten by remote and in-situ plasma generation. Besides relating only to etching, there is no involvement of active hydrogen species for abatement of fluoride contamination. Okano, U.S. Pat. No. 5,591,486 describes a method for depositing a film using a CVD deposition chamber with an attached remote chamber. One film precursor gas is introduced into the remote chamber to be activated by microwaves while a second precursor gas enters directly into the deposition chamber.